一、需求
Cesium支持加载gltf和3dtiles等三维数据模型，实现了很好的封装，往往只需要给一个uri就能加载模型文件，并实现贴图渲染等。但是好的封装带来的问题是如果开发者想要自定义贴图，那该怎么办？不得不从源码入手。
二、价值
这篇文章的价值不仅仅是gltf增加纹理贴图，因为刚才说到的3dtiles其实也是基于gltf来实现的模型，那么如果想给3dtiles增加自定义贴图，是否也意味着可以走gltf这条路，并且从gltf这层实现之后，是否意味着对b3dm/i3dm/cmpt等的统一。
三、源码解读
1.框架：
3dtiels中b3dm和i3dm是以gltf为基础进行加载和渲染的，因此Cesium在封装的B3dmLoader和I3dmLoader中都有调用_gltfLoader的地方，其中使用的是：gltfLoader.process
B3dmLoader.prototype.process = function(frameState) {
...
const ready = this._gltfLoader.process(frameState);
...
};
I3dmLoader.prototype.process = function(frameState) {
...
ready = gltfLoader.process(frameState);
...
};在这个函数中抛开异常处理逻辑，关键函数在于：loadResources5(this, frameState):
async function loadResources5(loader, frameState) {
//给出Json指引
const gltf = loader.gltfJson;
//具体的资源加载
const promise = parse(loader, gltf, supportedImageFormats, frameState);
...
//注意这里由于模型资源加载完成后是不需要中间数据的，为了减少内存的消耗，Cesium这里对Json信息进行了清理
if (defined_default(loader._gltfJsonLoader) && loader._releaseGltfJson) {
ResourceCache_default.unload(loader._gltfJsonLoader);
loader._gltfJsonLoader = void 0;
}
return promise;
}再来看parse：
function parse(loader, gltf, supportedImageFormats, frameState) {
//...拓展项相关的数据处理
//注意一下结构，实际就是按照Json的指引，将具体的数据请取出，熟悉gltf的Json项自然就明白了各项的含义
const nodes = loadNodes(loader, gltf, supportedImageFormats, frameState);
const skins = loadSkins(loader, gltf, nodes);
const animations = loadAnimations(loader, gltf, nodes);
const articulations = loadArticulations(gltf);
const scene = loadScene(gltf, nodes);
const components = new Components2();
const asset = new Asset2();
const copyright = gltf.asset.copyright;
...
//将取出的数据存储在components中
components.asset = asset;
components.scene = scene;
components.nodes = nodes;
...
loader._components = components;
...
}至此，数据的读取，处理就完成了，意味着渲染隐含与其中。下面将重点分析。
2.渲染逻辑
A.纹理资源的加载
如果了解gltf的管理方式：
不难看出node是总览全局的，那么进入 node处理部分：loadNodes::loadNode
function loadNode(loader, gltf, gltfNode, supportedImageFormats, frameState) {
...
//一个node对应一个meshId，用于获取对应的mesh
const meshId = gltfNode.mesh;
if (defined_default(meshId)) {
const mesh = gltf.meshes[meshId];
//mesh中又包括多个图元
const primitives = mesh.primitives;
const primitivesLength = primitives.length;
for (let i = 0; i < primitivesLength; ++i) {
node.primitives.push(
//图元是最小的渲染可调度单位
loadPrimitive(
loader,
gltf,
primitives[i],
defined_default(node.instances),
supportedImageFormats,
frameState
)
);
}
...
}
return node;
}在最小的渲染单位primitive中：
function loadPrimitive(loader, gltf, gltfPrimitive, hasInstances, supportedImageFormats, frameState) {
...
//从图元取得MaterialId
const materialId = gltfPrimitive.material;
if (defined_default(materialId)) {
//加载材质的入口，也意味着材质的管理（增删改）都可以从这里找到
primitive.material = loadMaterial(
loader,
gltf,
gltf.materials[materialId],
supportedImageFormats,
frameState
);
}
...
return primitive;
}加载材质部分主要包括：初始化一个空材质+往材质模板中填充数据
function loadMaterial(loader, gltf, gltfMaterial, supportedImageFormats, frameState) {
//首先进来的第一件事先创建一个空材质用于填充数据
const material = new Material3();
...
//直接计算的填充
material.unlit = defined_default(extensions.KHR_materials_unlit);
...
//针对特定类型材质特定参数计算，最后再填充
specularGlossiness.glossinessFactor = pbrSpecularGlossiness.glossinessFactor;
material.pbrSpecularGlossiness = pbrSpecularGlossiness;
...
//重头戏就是这里的加载纹理
metallicRoughness.baseColorTexture = loadTexture(
loader,
gltf,
pbrMetallicRoughness.baseColorTexture,
supportedImageFormats,
frameState
);
...
return material;
}加载纹理的逻辑：
function loadTexture(loader, gltf, textureInfo, supportedImageFormats, frameState, samplerOverride) {
//检查Image是否可用
//纹理加载器
const textureLoader = ResourceCache_default.getTextureLoader({
gltf,
textureInfo,
gltfResource: loader._gltfResource,
baseResource: loader._baseResource,
supportedImageFormats,
frameState,
asynchronous: loader._asynchronous
});
//纹理解释器
const textureReader = GltfLoaderUtil_default.createModelTextureReader({
textureInfo
});
//将相关加载放入总加载器管理
loader._textureLoaders.push(textureLoader);
...
loader._textureState = GltfLoaderState.FAILED;
loader._textureErrors.push(error);
loader._texturesPromises.push(promise);
loader._textureCallbacks[index]...
return textureReader;
}B.纹理资源应用
当纹理加载完成，就要考虑如何消费纹理，即编写shader和处理：
纹理的使用往往是在FragmentShader中，这块的编码在Cesium中为：
var MaterialStageFS_default = "// If the style color is white, it implies the feature has not been styled.\nbool isDefaultStyleColor(vec3 color)\n{\n return all(greaterThan(color, vec3(1.0 - czm_epsilon3)));\n}\n\nvec3 blend(vec3 sourceColor, vec3 styleColor, float styleColorBlend)\n{\n vec3 blendColor = mix(sourceColor, styleColor, styleColorBlend);\n vec3 color = isDefaultStyleColor(styleColor.rgb) ? sourceColor : blendColor;\n return color;\n}\n\nvec2 computeTextureTransform(vec2 texCoord, mat3 textureTransform)\n{\n return vec2(textureTransform * vec3(texCoord, 1.0));\n}\n\n#ifdef HAS_NORMALS\nvec3 computeNormal(ProcessedAttributes attributes)\n{\n // Geometry normal. This is already normalized \n vec3 ng = attributes.normalEC;\n\n vec3 normal = ng;\n #if defined(HAS_NORMAL_TEXTURE) && !defined(HAS_WIREFRAME)\n vec2 normalTexCoords = TEXCOORD_NORMAL;\n #ifdef HAS_NORMAL_TEXTURE_TRANSFORM\n normalTexCoords = computeTextureTransform(normalTexCoords, u_normalTextureTransform);\n #endif\n\n // If HAS_BITANGENTS is set, then HAS_TANGENTS is also set\n #ifdef HAS_BITANGENTS\n vec3 t = attributes.tangentEC;\n vec3 b = attributes.bitangentEC;\n mat3 tbn = mat3(t, b, ng);\n vec3 n = texture(u_normalTexture, normalTexCoords).rgb;\n normal = normalize(tbn * (2.0 * n - 1.0));\n #elif (__VERSION__ == 300 || defined(GL_OES_standard_derivatives))\n // If derivatives are available (not IE 10), compute tangents\n vec3 positionEC = attributes.positionEC;\n vec3 pos_dx = dFdx(positionEC);\n vec3 pos_dy = dFdy(positionEC);\n vec3 tex_dx = dFdx(vec3(normalTexCoords,0.0));\n vec3 tex_dy = dFdy(vec3(normalTexCoords,0.0));\n vec3 t = (tex_dy.t * pos_dx - tex_dx.t * pos_dy) / (tex_dx.s * tex_dy.t - tex_dy.s * tex_dx.t);\n t = normalize(t - ng * dot(ng, t));\n vec3 b = normalize(cross(ng, t));\n mat3 tbn = mat3(t, b, ng);\n vec3 n = texture(u_normalTexture, normalTexCoords).rgb;\n normal = normalize(tbn * (2.0 * n - 1.0));\n #endif\n #endif\n\n #ifdef HAS_DOUBLE_SIDED_MATERIAL\n if (czm_backFacing()) {\n normal = -normal;\n }\n #endif\n\n return normal;\n}\n#endif\n\nvoid materialStage(inout czm_modelMaterial material, ProcessedAttributes attributes, SelectedFeature feature)\n{\n #ifdef HAS_NORMALS\n material.normalEC = computeNormal(attributes);\n #endif\n\n vec4 baseColorWithAlpha = vec4(1.0);\n // Regardless of whether we use PBR, set a base color\n #ifdef HAS_BASE_COLOR_TEXTURE\n vec2 baseColorTexCoords = TEXCOORD_BASE_COLOR;\n\n #ifdef HAS_BASE_COLOR_TEXTURE_TRANSFORM\n baseColorTexCoords = computeTextureTransform(baseColorTexCoords, u_baseColorTextureTransform);\n #endif\n\n baseColorWithAlpha = czm_srgbToLinear(texture(u_baseColorTexture, baseColorTexCoords));\n\n #ifdef HAS_BASE_COLOR_FACTOR\n baseColorWithAlpha *= u_baseColorFactor;\n #endif\n #elif defined(HAS_BASE_COLOR_FACTOR)\n baseColorWithAlpha = u_baseColorFactor;\n #endif\n\n #ifdef HAS_POINT_CLOUD_COLOR_STYLE\n baseColorWithAlpha = v_pointCloudColor;\n #elif defined(HAS_COLOR_0)\n vec4 color = attributes.color_0;\n // .pnts files store colors in the sRGB color space\n #ifdef HAS_SRGB_COLOR\n color = czm_srgbToLinear(color);\n #endif\n baseColorWithAlpha *= color;\n #endif\n\n material.diffuse = baseColorWithAlpha.rgb;\n material.alpha = baseColorWithAlpha.a;\n\n #ifdef USE_CPU_STYLING\n material.diffuse = blend(material.diffuse, feature.color.rgb, model_colorBlend);\n #endif\n\n #ifdef HAS_OCCLUSION_TEXTURE\n vec2 occlusionTexCoords = TEXCOORD_OCCLUSION;\n #ifdef HAS_OCCLUSION_TEXTURE_TRANSFORM\n occlusionTexCoords = computeTextureTransform(occlusionTexCoords, u_occlusionTextureTransform);\n #endif\n material.occlusion = texture(u_occlusionTexture, occlusionTexCoords).r;\n #endif\n\n #ifdef HAS_EMISSIVE_TEXTURE\n vec2 emissiveTexCoords = TEXCOORD_EMISSIVE;\n #ifdef HAS_EMISSIVE_TEXTURE_TRANSFORM\n emissiveTexCoords = computeTextureTransform(emissiveTexCoords, u_emissiveTextureTransform);\n #endif\n\n vec3 emissive = czm_srgbToLinear(texture(u_emissiveTexture, emissiveTexCoords).rgb);\n #ifdef HAS_EMISSIVE_FACTOR\n emissive *= u_emissiveFactor;\n #endif\n material.emissive = emissive;\n #elif defined(HAS_EMISSIVE_FACTOR)\n material.emissive = u_emissiveFactor;\n #endif\n\n #if defined(LIGHTING_PBR) && defined(USE_SPECULAR_GLOSSINESS)\n #ifdef HAS_SPECULAR_GLOSSINESS_TEXTURE\n vec2 specularGlossinessTexCoords = TEXCOORD_SPECULAR_GLOSSINESS;\n #ifdef HAS_SPECULAR_GLOSSINESS_TEXTURE_TRANSFORM\n specularGlossinessTexCoords = computeTextureTransform(specularGlossinessTexCoords, u_specularGlossinessTextureTransform);\n #endif\n\n vec4 specularGlossiness = czm_srgbToLinear(texture(u_specularGlossinessTexture, specularGlossinessTexCoords));\n vec3 specular = specularGlossiness.rgb;\n float glossiness = specularGlossiness.a;\n #ifdef HAS_SPECULAR_FACTOR\n specular *= u_specularFactor;\n #endif\n\n #ifdef HAS_GLOSSINESS_FACTOR\n glossiness *= u_glossinessFactor;\n #endif\n #else\n #ifdef HAS_SPECULAR_FACTOR\n vec3 specular = clamp(u_specularFactor, vec3(0.0), vec3(1.0));\n #else\n vec3 specular = vec3(1.0);\n #endif\n\n #ifdef HAS_GLOSSINESS_FACTOR\n float glossiness = clamp(u_glossinessFactor, 0.0, 1.0);\n #else\n float glossiness = 1.0;\n #endif\n #endif\n\n #ifdef HAS_DIFFUSE_TEXTURE\n vec2 diffuseTexCoords = TEXCOORD_DIFFUSE;\n #ifdef HAS_DIFFUSE_TEXTURE_TRANSFORM\n diffuseTexCoords = computeTextureTransform(diffuseTexCoords, u_diffuseTextureTransform);\n #endif\n\n vec4 diffuse = czm_srgbToLinear(texture(u_diffuseTexture, diffuseTexCoords));\n #ifdef HAS_DIFFUSE_FACTOR\n diffuse *= u_diffuseFactor;\n #endif\n #elif defined(HAS_DIFFUSE_FACTOR)\n vec4 diffuse = clamp(u_diffuseFactor, vec4(0.0), vec4(1.0));\n #else\n vec4 diffuse = vec4(1.0);\n #endif\n czm_pbrParameters parameters = czm_pbrSpecularGlossinessMaterial(\n diffuse.rgb,\n specular,\n glossiness\n );\n material.diffuse = parameters.diffuseColor;\n // the specular glossiness extension's alpha overrides anything set\n // by the base material.\n material.alpha = diffuse.a;\n material.specular = parameters.f0;\n material.roughness = parameters.roughness;\n #elif defined(LIGHTING_PBR)\n #ifdef HAS_METALLIC_ROUGHNESS_TEXTURE\n vec2 metallicRoughnessTexCoords = TEXCOORD_METALLIC_ROUGHNESS;\n #ifdef HAS_METALLIC_ROUGHNESS_TEXTURE_TRANSFORM\n metallicRoughnessTexCoords = computeTextureTransform(metallicRoughnessTexCoords, u_metallicRoughnessTextureTransform);\n #endif\n\n vec3 metallicRoughness = texture(u_metallicRoughnessTexture, metallicRoughnessTexCoords).rgb;\n float metalness = clamp(metallicRoughness.b, 0.0, 1.0);\n float roughness = clamp(metallicRoughness.g, 0.04, 1.0);\n #ifdef HAS_METALLIC_FACTOR\n metalness *= u_metallicFactor;\n #endif\n\n #ifdef HAS_ROUGHNESS_FACTOR\n roughness *= u_roughnessFactor;\n #endif\n #else\n #ifdef HAS_METALLIC_FACTOR\n float metalness = clamp(u_metallicFactor, 0.0, 1.0);\n #else\n float metalness = 1.0;\n #endif\n\n #ifdef HAS_ROUGHNESS_FACTOR\n float roughness = clamp(u_roughnessFactor, 0.04, 1.0);\n #else\n float roughness = 1.0;\n #endif\n #endif\n czm_pbrParameters parameters = czm_pbrMetallicRoughnessMaterial(\n material.diffuse,\n metalness,\n roughness\n );\n material.diffuse = parameters.diffuseColor;\n material.specular = parameters.f0;\n material.roughness = parameters.roughness;\n #endif\n}\n";相当的长，但是这中间有上述分析过程中对材质单个参数（粗糙度，金属度）和纹理的处理，不妨一读。那么这段Shader如何使用的呢？
MaterialPipelineStage.process = function(renderResources, primitive, frameState) {
...
processMaterialUniforms(
material,
uniformMap2,
shaderBuilder,
defaultTexture,
defaultNormalTexture,
defaultEmissiveTexture,
disableTextures
);
if (defined_default(material.specularGlossiness)) {
processSpecularGlossinessUniforms(
material,
uniformMap2,
shaderBuilder,
defaultTexture,
disableTextures
);
}
else {
processMetallicRoughnessUniforms(
material,
uniformMap2,
shaderBuilder,
defaultTexture,
disableTextures
);
}
...
shaderBuilder.addFragmentLines(MaterialStageFS_default);
...
};是的，直接在最下方添加到ShaderBuilder。
看上去我们这个过程从数据获取到消费似乎是完成了，但是细心的人应该发现了MaterialStageFS_default 还有一些宏或者不同的纹理它的采样器uv这样的数据其实也是要告知shader的，那么这种处理实际是在processGldLightMapUniforms::processTexture2这个函数中：
function processTexture2(shaderBuilder, uniformMap2, textureReader, uniformName, defineName, defaultTexture) {
//添加Uniform变量
shaderBuilder.addUniform(
"sampler2D",//类型
uniformName,//变量名
ShaderDestination_default.FRAGMENT//添加到Fragment
);
uniformMap2[uniformName] = function() {
return defaultValue_default(textureReader.texture, defaultTexture);
};
//shaderBuilder.addDefine用于在Shader中定义变量并给初值
//(名称，默认值，添加位置)
const textureDefine = `HAS_${defineName}_TEXTURE`;//宏
shaderBuilder.addDefine(textureDefine, void 0, ShaderDestination_default.FRAGMENT);
const texCoordIndex = textureReader.texCoord;
const texCoordVarying = `v_texCoord_${texCoordIndex}`;
const texCoordDefine = `TEXCOORD_${defineName}`;
//uv
shaderBuilder.addDefine(
texCoordDefine,
texCoordVarying,
ShaderDestination_default.FRAGMENT
);
...
}至此，整个流程才算完成，理解了以上流程之后，要想加一张纹理那就比较容易了。
3.实操添加一张贴图
这里给出步骤思路，具体实现自己写一遍应该会好很多：
a.新增一张纹理贴图，意味着Material要加新成员，对应的是loadMaterial中的空材质构造函数：
const material = new Material3();b. 对空material填充需要loadTexture，这里要注意纹理解释器的丰富，封装在了getAllTextureReaders中；
c.加载好纹理之后就是纹理处理，也就是shader部分，这里一共又可以分为两步：
processTexture2添加uniform数据资源，往shader压入变量及其值；文章来源：https://www.toymoban.com/news/detail-460625.html
编写shader代码：MaterialStageFS_default。直接在这里改就可以利用上ShaderBuilder的添加一步到位。 文章来源地址https://www.toymoban.com/news/detail-460625.html
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